The manufacture of grain oriented electrical steels requires critical control of chemistry and processing to achieve the desired magnetic properties in a stable and reproducible manner. The present invention produces excellent magnetic properties in (110)[001] oriented electrical steel having less than 0.005% Al using a single cold reduction stage.
Grain oriented electrical steels are characterized by the level of magnetic properties developed, the grain growth inhibitors used and the processing steps which provide these properties. Regular (or conventional) grain oriented electrical steels typically have magnetic permeability below 1880 as measured at 796 A/m. Regular grain oriented electrical steels are typically produced using manganese and sulfur (and/or selenium) as the principle grain growth inhibitor(s) with two cold reduction steps separated by an annealing step. Aluminum (less than 0.005%), antimony, boron, copper, nitrogen and other elements are sometimes present and may supplement the manganese sulfide/selenide inhibitor(s) to provide grain growth inhibition.
Representative processes for producing regular grain oriented electrical steel are taught in U.S. Pat. Nos. 3,764,406; 3,843,422; 4,202,711 and 5,061,326 which are incorporated herein by reference. Regular grain oriented electrical steel strip or sheet is generally produced using two stages of cold reduction in order to achieve the desired magnetic properties. While a single stage cold reduction process has long been sought since two or more processing steps (at least one cold rolling stage and an intermediate anneal) are eliminated, the magnetic properties have lacked the desired level of consistency and quality.
Regular grain oriented electrical steel may have a mill glass film, commonly called forsterite, or an insulative coating, commonly called a secondary coating, applied over or in place of the mill glass film, or may have a secondary coating designed for punching operations where laminations free of mill glass coating are desired in order to avoid excessive die wear. Generally, magnesium oxide is applied onto the surface of the steel prior to the high temperature anneal which serves as an annealing separator coating. These coatings also influence the development and stability of secondary grain growth during the final high temperature anneal, react to form the forsterite (or mill glass) coating on the steel and effect desulfurization of the base metal during annealing.
To obtain material having a high degree of cube-on-edge orientation, the material must have a structure of recrystallized grains with the desired orientation prior to the high temperature portion of the final anneal and must have grain growth inhibition to restrain primary grain growth in the final anneal until secondary grain growth occurs. Of great importance in the development of the magnetic properties of electrical steel is the vigor and completeness of secondary grain growth. This depends on having a fine dispersion of manganese sulfide (or other) inhibitors which is capable of restraining primary grain growth in the temperature range of 535.degree.-925.degree. C. (1000.degree.-1700.degree. F.). Thereafter, the cube-on-edge nuclei have sufficient energy to develop into large secondary crystals which grow at the expense of the less perfectly oriented matrix of primary grains. The dispersion of manganese sulfide is typically provided by high temperature slab or ingot reheating prior to hot rolling during which the fine manganese sulfide is precipitated.
The production of cube-on-edge oriented electrical steel requires that the material be heated to a temperature which dissolves the inhibitor prior to hot rolling so that during hot rolling the inhibitor is precipitated as small, uniform particles. U.S. Pat. No. 2,599,340 disclosed the basic process for the production of material from ingots and U.S. Pat. Nos. 3,764,406 and 4,718,951 obtained good magnetic properties from material which was continuously cast as slab followed by heating and hot rolling the cast slab prior to the conventional hot rolling step to reduce the size of the columnar grain structure.
Work done in the past, as represented in U.S. Pat. No. 3,333,992 (incorporated herein by reference), added large amounts of sulfur during the early portion of the final high temperature anneal by providing a sulfur-bearing annealing atmosphere or surface coating or both. However, achieving permeabilities at 796 A/m consistently in excess of 1800 required at least two cold reduction stages separated by an annealing step.
U.S. Pat. No. 4,493,739 teaches a method for producing regular grain oriented electrical steel using one or two stages of cold rolling. This patent teaches the use of 0.02-0.2% copper in combination with control of the hot mill finishing temperature to improve the uniformity of the magnetic properties. Phosphorus was controlled to less than 0.01% to reduce inclusions. Tin up to 0.10% could be employed to improve core loss of the finished grain oriented electrical steel by reducing the size of the (110)[001] grains. The manganese sulfide precipitates were considered to be weak and the uniformity of the magnetic properties were improved by forming fine copper sulfide precipitates to supplement the manganese sulfide inhibitor. During hot roll finishing, the entrance and exit temperatures were controlled to be from 1000.degree.-1250.degree. C. and 900.degree.-1150.degree. C., respectively. All of the examples of U.S. Pat. No. 4,493,739 show a conventional two stage cold rolling process. The manganese/copper sulfide precipitates formed after hot rolling were fine and uniformly dispersed and heavy 60-80% cold reductions were required for grain size control and texture development. U.S. Pat. No. 4,493,739 implied that unstable secondary recrystallization would result with a single stage of cold reduction process.
U.S. Pat. No. 3,986,902 is related to excess manganese in regular grain oriented electrical steel. The patent uses manganese sulfide for the grain growth inhibitor. Hot working causes these precipitates to grow appreciably and to be concentrated intergranularly such that the precipitates are less effective as grain growth inhibitors. It is therefore essential that the precipitates be dissolved in solid solution and that they precipitate as finely dispersed particles during or after the final step of hot rolling to band. Prior art practices discussed in this patent reviewed the need to provide a silicon steel with 0.07-0.11% manganese and 0.02-0.4% sulfur to provide the necessary grain growth inhibitors. Manganese in excess of that required to combine with sulfur to form manganese sulfide was present. The excess manganese was desired to prevent hot shortness. However, higher excess manganese decreased the solubility of manganese sulfide and required higher slab or ingot reheating temperatures to dissolve the manganese sulfide. The patent sought to lower reheating temperatures to 1250.degree. C. (2290.degree. F.) or less by reducing the solubility product to a maximum of about 0.0012%. Effective grain growth inhibition with less manganese sulfide required lowering the levels of insoluble oxides, such as Al.sub.2 O.sub.3, MnO, etc., in the steel. It was believed that the oxides had very low solubility in solid steel, particularly at the lower reheating temperatures desired by this invention. Sulfur also had a tendency to react with the oxide inclusions and form oxysulfides, negatively influencing the solubility limits and affecting the development of the desired cube-on-edge orientation. The oxide inclusions noted in U.S. Pat. No. 3,986,902 were incurred during melting and teeming.
Various attempts have been made to reduce the oxygen content to minimize such inclusions. U.S. Pat. No. 3,802,937 used lower amounts of manganese sulfide and minimized oxide nucleation by protecting the pouring stream during teeming to avoid reoxidation. The patent required that the manganese sulfide solubility product be maintained at less than 0.0012% and preferably from 0.0007-0.0010%. This was accomplished, for example, by using 0.05% manganese and 0.02% sulfur. Reducing either sulfur, manganese or both served to provide a lower solubility product; however, since the sulfur must be removed in the final anneal, it was preferred to keep sulfur low and maintain a controlled level of manganese. This resulted in a process having about 0.07-0.08% manganese and about 0.011-0.015% sulfur. The excess manganese content insured that all of the sulfur was combined as manganese sulfide. As previously mentioned, control of the reoxidation products enabled lower levels of manganese and sulfur with the lower slab reheating temperatures. Lower manganese-to-sulfur ratios (about 1.7) could be used while avoiding hot brittleness as compared with previous practices in the art which required ratios of about 3.0. Per the teachings of U.S. Pat. No. 3,802,937, the slabs were reheated to a temperature of less than 1260.degree. C. (2300.degree. F.) and hot rolled to 1.3-2.5 mm (0.05-0.10 inch) thickness before the temperature fell to between 790.degree.-950.degree. C. (1450.degree.-1750.degree. F.). After hot rolling, the steel is cooled to between 450.degree.-560.degree. C. (850.degree.-1050.degree. F.) prior to coiling. Annealing of the hot rolled bands at a temperature of at least 980.degree. C. (1800.degree. F.) was preferred but optional. The bands were cold reduced to an intermediate thickness, annealed and again cold reduced to a typical final thickness of about 0.28 mm (0.011 inch). The steel was then decarburized at a temperature of 760.degree.-81 5.degree. C. (1400.degree.-1500.degree. F.) to reduce the carbon to 0.007% or less and provide primary recrystallization and subjected to a final anneal at about 1065.degree.-1175.degree. C. (1950.degree.-2150.degree. F.) to effect secondary recrystallization. The one example used 0.031% carbon, 0.055% manganese, 0.006% phosphorus, 0.02% sulfur, 2.97% silicon, 0.002% aluminum, 0.005% nitrogen and balance iron.
A method to produce a regular grain oriented electrical steel using a single cold reduction step as one of the processing steps is taught in USSN 07/974,772 (incorporated herein by reference). This patent application discloses the use of uncombined manganese below 0.024%, the addition of sulfur in the annealing separator and the control of the austenite volume fraction at 1150.degree. C. to at least 7% to enable the use of a single stage cold reduction.
The chemistry for regular grain oriented electrical steel having a manganese sulfide inhibitor system has typically restricted the level of chromium to about 0.06% maximum (see U.S. Pat. No. 3,986,902; column 5, line 47) as an accepted commercial specification.
The addition of chromium in high permeability electrical steel has been in small amounts for supplementing an aluminum nitride inhibitor system or larger amounts when used as a coating additive, such as chromic acid. An example of chromium being used to supplement the aluminum nitride inhibitor system is WO 9313236 where chromium ranged from 0.02-0.12%. Several Japanese patent applications (Japanese 02200731; 02200732; and 02200733) relating to high permeability electrical steel having an aluminum nitride inhibitor system taught the addition of chromium from 0.07-0.25% in a composition of 2.0-4.0% silicon, 0.025-0.095% carbon, 0.08-0.45% manganese, 0.015% max sulfur, 0.01-0.06% aluminum, 0.003-0.0130% nitrogen, 0.005-0.045% phosphorus and up to 1.5% molybdenum, vanadium, niobium, antimony, tin, titanium, tellurium and/or boron. U.S. Pat. Nos. 4,824,493 and 4,692,193 teach the addition of up to 0.4% chromium to high permeability electrical steel made using aluminum nitride as the grain growth inhibitor.
Large additions of chromium have been added to oriented low alloy steels having less than 2% silicon as evidenced in U.S. Pat. No. 4,251,296. The sum of chromium and silicon is less than 2% and typically about 1.2%.
The addition of chromium to the surface of the steel to influence the forsterite reaction or introduce strain on the base metal for domain refinement has been previously disclosed. U.S. Pat. Nos. 4,909,864 and 4,985,635 teach the coating of chromium carbides and nitrides on the polished surface of electrical steel following the final anneal to provide domain refinement. When used as a forsterite reaction inhibitor, lines of chromium metal may be plated onto the surface to be finally annealed in combination with a high energy beam (such as provided by a laser treatment) to diffuse the inhibitors into the iron base matrix after cold rolling. U.S. Pat. No. 4,032,366 teaches the addition of 0.3-6.0% hexavalent chromium to a magnesia slurry applied onto the surface of a grain oriented electrical steel.
Tin has been added to high permeability electrical steel having an aluminum nitride inhibitor system for different reasons. Japanese Patent Publication No. 53-134722 has an aluminum nitride inhibitor system and adds tin in the range of 0.1-0.5% to reduce the size of the secondary recrystallized grains. U.S. Pat. No. 5,049,205 teaches the addition of tin in lower amounts (0.01-0.10%) for a nitriding process after completion of primary recrystallization to increase the efficiency of the nitriding process in an aluminum nitride inhibitor system. Tin was recognized as reducing the amount of oxygen after decarburizing and making the sheet less susceptible to the dew point. This also contributed to more stable magnetic properties since a low dew point is difficult to maintain. U.S. Pat. No. 4,992,114 adds 0.05-0.25% tin to an aluminum nitride inhibited electrical steel. With less than 0.05% tin, the secondary recrystallization becomes unstable.
As pointed out by the above patents, the control of the manganese sulfide precipitates and the various processing steps required for producing regular grain oriented electrical steel having uniform and consistent magnetic properties is difficult. The ability to obtain the desired properties in regular grain oriented electrical steel having less than 0.005% aluminum using a single cold reduction process is even more difficult and it is this challenge to which the present invention is directed.